Team:TU Darmstadt/Project/Background

Background

In this year's project, we focused on the biological production of the building blocks, the so called monomers, of the polymers PLGA (Poly(lactic-co-glycolic-acid)) and PLGC (Poly(lactide-co-glycolide-co-caprolactone)). The needed monomers for these polymers are lactic acid, glycolic acid and -caprolactone. Since some former iGEM Teams, e.g. the iGEM-team from Evry in 2016, already worked with lactic acid, we focused on producing -caprolactone and glycolic acid. Another reason why we did not focus on lactic acid is that it is already industrially produced in large scale by bacterial fermentation of sugars obtained from renewable resources which makes it an eco-friendly product. [1]

For the production of these two compounds, currently a lot of harmful chemicals are needed. Glycolic acid can be produced in two different manners either it is produced by an alkaline hydrolysis of chloric acid or it is synthesized out of water, carbon monoxide and formaldehyde. [2]

Formaldehyde and chloric acid are both toxic for humans and may harm exposed persons long-term. Furthermore, most of the chemicals needed for the production of glycolic acid are obtained from petrochemicals. Petrochemicals are derived from petroleum or other fossil fuel, e.g. natural gas and coal. The reservoirs for these are finite and it is therefore concerning that the society is depending on them. Additionally, for the retrieval of them, nature often has to be destroyed and during the retrieval gases and aerosol are released, which may cause cancer. Another problem with fossil oil especially is that it is often drilled illegally which increases the negative impact on the environment by e.g. destroying soil. -caprolactone is produced in a multi-10000 ton scale per year from cyclohexanol, acetic acid and hydrogen peroxide.[3] Acetic acid is mostly also derived from petrochemicals . Hydrogen peroxide and cyclohexanone are also toxic for humans and hydrogen peroxide is also environmentally harmful. Furthermore, hydrogen peroxide has only a modest selectivity (80-90%) which leads to a loss of product.

To overcome the named problems and to establish an environmentally friendly and safe way with high yields to produce the named monomer is the goal of this year's iGEM project. In order to be competitive with the chemical production, we need to achieve high production yields. We choose Escherichia coli and Saccharomyces cerevisiae as host organisms to allow a comparison and to draw conclusions which one is better suited for large scale production of monomers. What is PLGA and what is PLGC you might wonder. As many other common plastics like nylon or polyethylenterephthalate (PET), PLGA and PLGC belong to the family of polyesters. Both are built from lactic acid and glycolic acid monomers. PLGC additionally contains -caprolactone. PLGA identifies through high mechanical resistance and simultaneous thermoplastic behavior. Our monomers only consist of small carbon chains and the amount of dipol-dipol-interactions is high which leads to strong intermolecular forces which makes PLGA hard and mechanical resistant under normal conditions. By heating the polymer up, it is possible to overcome these forces, which leads to an increase of flexibility up to melting of the polymer. The melting process is called glass transition range.

As PLGA, PLGC is also a thermoplast. The structural different is that PLGC additionally includes -caprolactone which has also two reactive groups. Nevertheless, both polymers have differences in their properties. PLGC has a lower mechanical resistance because of the caprolactone. Since the caprolactone adds 6 linear carbon atoms the frequency of the ester groups is lower and for this reason the amount of dipole-dipole interactions is lower as well. This leads to a lower glass transition range.

Why would we be interested in PLGA and PLGC?

You might already know PLA (poly-lactic-acid) made out of lactic acid, which is an established biodegradable polymer and has a long life time. It is used e.g. for packing materials and disposable cups. (see figure XX, CHART) PLGA is made out of lactic acid and additionally glycolic acid. Glycolic acid speeds up the degradation process and with the ratio of glycolic acid to lactic acid the degradation time can be varied. The -caprolactones which is needed additionally for PLGC further modifies the degradation behavior of the polymer by making the ester bonds easier accessible for water, which speeds up de degradation process. The amount of caprolactone can therefore influence how fast or slow the polymer gets degraded. 

[4]

This makes PLGA and PLGC suitable for medical application such as implants, tissue engineering and drug delivery systems where the polymercan degrade in a predetermined timeframe depending on the application from weeks to months. 

[5] PLGA degrades into its building blocks in the body which can be metabolized. It is therefore approved by the FDA for clinical usage. 
[6]

When PLGA and PLGC are degrading, they undergo surface and bulk erosion depending on the size. Surface erosion means that the polymer degrades from the outside to the inside while during bulk erosion water diffuses inside and it degrades at almost the same rate in every place in the polymer. When the polymer is bigger it first undergoes surface erosion and when it gets smaller it changes to bulk erosion. PLGA and PLGC catalyze their degradation by decreasing the pH while degrading since they degrade into acids. This has also an impact on implants and nanospheres in a humans even though the blood maintains a constant pH. When they reach or have a certain size bluc erosion takes due to water diffusion into the polymer. The nanoosphere or implant is now degrading also from the inside which leads to a decline of the pH inside and a katalysation of the degradation. Due to this fact their degradation is not linear. 

[7]

Another polymer which can be produced with -caprolactone is PCL (poly-caprolactone). This has a longer live time than PLGC since it does not autocatalysis its degradation. This makes this polymer suitable for long-term medication via nanospheres as well as implants. [8]


Another problem we are facing today, is the amount of microplastics found in our environment, especially in our oceans. Microplastics are plastic particles with a diameter of less than 5 mm which are released by e.g. plastic trash and cosmetic products. Microplastics enter the nutrition chain via water organisms, which in the end are eaten by us humans. Furthermore, a lot of plastics incorporate substances like softening agents which can potentially cause cancer. Since our polymer degrades in water, it does not cause these problems and is therefore an ecofriendly option.
  1. ↑ Komesu, A., Oliveira, BioRes. 12(2). 4364-4383, 2017
  2. ↑ patents.google [1]
  3. ↑ Sandy Schmidt, Angewandte Chemie, Wiley Online Library, 2015.
  4. ↑ Degradation behavior PLGC [2]
  5. ↑ Qing Cai, Polymers for advanced technologies 13, 105-11, 2002
  6. ↑ FDA decision about PLGA https://www.millioninsights.com/industry-reports/poly-lactic-co-glycolic-acid-plga-market?utm_source=pressrelease&utm_medium=referral&utm_campaign=Abnewswire_Shweta_Sept25&utm_content=Content
  7. ↑ Friederike von Burkersroda, Biomaterials 23 (2002) 4221–4231, 2002
  8. ↑ Adam L. Sisson, Elsevier, 2013